1. Field of the Invention
The present invention relates to a control device for a machine tool including a feed shaft motor for driving a feed shaft and a main shaft motor for driving a main shaft, and more particularly to a control device for a machine tool configured such that AC power supplied from the AC power supply side is converted into DC power to be output, and thereafter, the DC power is converted into AC power for driving a motor so as to supply the AC power to the feed shaft motor and to the main shaft motor for driving.
2. Description of the Related Art
In a machine tool including a feed shaft motor and a main shaft motor, the main shaft motor is used as a driving source for driving a main shaft mounted with a tool (various tools), and the feed shaft motor is used as a driving source for driving a feed shaft configured to move the main shaft or a workpiece to be processed. In the machine tool as described above, in view of control feasibility, AC power input from the three-phase AC power supply side is temporarily converted into DC power, and thereafter, the DC power is further converted into AC power, and the AC power is used for driving a motor provided for each of drive shafts (a main shaft and a feed shaft).
A control device provided in a machine tool is provided with, as a main circuit, a converter converting (rectifying) AC power supplied from the three-phase AC power supply side for outputting DC power, and inverters connected to a DC link (a direct-current link) as the DC side of the converter, and mutually converting power between DC power of the DC link, and AC power as driving electric power or regenerative electric power of a motor. Causing the control device to control AC output from each of the inverters at an intended voltage and at an intended frequency makes it possible to control the speed, the torque of a main shaft motor and a feed shaft motor connected to the AC side of each of the inverters, or the position of a rotor.
Regarding inverters, taking requirements for energy saving into consideration, many inverters are capable of regenerating electric power in order to store regenerative electric power generated at the time of motor deceleration in an electric storage device provided in a DC link for reuse as driving electric power of a motor, or to return the regenerative electric power to the AC power supply side.
On the other hand, regarding a converter, it is often the case that one converter is provided with respect to a plurality of inverters for the purpose of reducing the cost or the installation space of a motor control device in a machine tool. Further, as with the case of the inverters, the converter may also be capable of regenerating electric power, specifically, capable of returning regenerative energy generated at the time of motor deceleration to the AC power supply side, when taking requirements for energy saving into consideration.
When power failure occurs on the AC power supply side of a converter in a motor control device, in the aforementioned motor control device, it is not possible to continue a normal operation of a feed shaft motor and a main shaft motor. In this case, due to collision of a feed shaft, a motor, the motor control device for driving the motor, a tool connected to the motor to be driven by the motor control device, a workpiece to be processed by the tool, a production line including the motor control device and the like may cause trouble such as damage or deformation.
In order to prevent collision of a feed shaft due to power failure on the AC power supply side, it is necessary to stop the operation of the feed shaft motor for driving the feed shaft as soon as possible. In view of the above, a power failure determination unit is provided on the AC power supply side of a rectifier to monitor the presence or absence of power failure on the AC power supply side. At the time of power failure, a deceleration command is issued to the feed shaft motor so as to stop the feed shaft motor in order to avoid the aforementioned trouble or minimize the trouble. In this way, there is performed a protection operation of protecting a main shaft being moved by a feed shaft motor, a tool connected to the motor, or a workpiece to be processed by the tool. When the power supply of a computer unit of the control device is backed up by an uninterruptible power system (UPS) or the like, even when power failure occurs on the AC power supply side, the control device is capable of issuing a command, to the feed shaft motor inverter, indicating an operation to be taken in an emergency. It is possible to operate the feed shaft motor inverter for a while by the electric charges accumulated in a capacitor provided in the converter, and thereby possible to emergency-stop the feed shaft motor.
As described in Japanese Laid-open Patent Publication No. H7-143780, as a method for emergency-stopping a motor at the time of power failure on the AC power supply side, there is proposed a method for stopping a motor at an early stage by actively generating a reverse torque in the course of decelerating the motor.
However, when applying the technology, for example, described in Japanese Laid-open Patent Publication No. H7-143780, in which a deceleration command is issued to a feed shaft motor in association with power failure detection on the AC power supply side to emergency-stop the feed shaft motor to a motor drive device having a function of regenerating regenerative electric power generated at the time of motor deceleration on the AC power supply side, it is not possible to return the regenerative electric power to the AC power supply side at the time of power failure. As a result, the DC voltage of a DC link between the converter and the inverter rises. In particular, the above drawback is noticeable when regenerative electric power of a motor is large. In view of the above, typically, an inverter issues an “overvoltage alarm” for protecting the inverter itself, when the DC voltage of the DC link at the DC side of the inverter becomes excessively large, and the control is abandoned. In this case, it is incapable of emergency-stopping the motor by actively generating a reverse torque in the course of deceleration. As a result, it takes time until the motor stops after power failure on the AC power supply side, which is a problem. When the above problem occurs in aforementioned feed shaft motor, for example, it is not possible to avoid collision of a feed shaft.
Further, there is a case that is necessary to continue to supply driving electric power from a feed shaft motor inverter to a feed shaft motor, even when the feed shaft motor is decelerated, depending on the characteristics of the feed shaft motor, or the state of friction applied to the feed shaft to be driven by the feed shaft motor. Specifically, in this case, even when the feed shaft motor is being decelerated, regenerative electric power is not generated in the feed shaft motor. Therefore, the feed shaft motor inverter does not supply energy to the DC link. Contrary to the above, the feed shaft motor inverter converts DC power of the DC link into AC power, and supplies the AC power to the feed shaft motor. When power failure occurs on the AC power supply side in this condition, and a deceleration command for an emergency-stop as described above is issued, the DC voltage of the DC link is rapidly lowered. Typically, the inverter issues an “undervoltage alarm”, because an excessively low DC voltage of the DC link at the DC side of the inverter makes the inverter incapable of supplying electric power for driving. Thus, the control is abandoned. In this case, it is incapable of emergency-stopping the motor by actively generating a reverse torque in the course of deceleration. As a result, it takes time until the motor stops after power failure on the AC power supply side, which is a problem. When the above problem occurs in the aforementioned feed shaft motor, for example, it is not possible to avoid collision of a feed shaft.
In order to avoid these drawbacks, a configuration may be assumed, in which the DC voltage of a DC link is monitored, and when the DC voltage rises, a main shaft motor is accelerated for energy consumption by the amount corresponding to an increase in DC power of the DC link, which is a cause for the increase in the DC voltage of the DC link, whereby the increase in the DC voltage is suppressed. On the other hand, when the DC voltage of the DC link falls, regenerative electric power to be generated by decelerating the main shaft motor is supplied for compensation by the amount corresponding to a decrease in DC power of the DC link, which is a cause for the decrease in the DC voltage of the DC link, whereby the decrease in DC power of the DC link is suppressed. When the main shaft motor is an induction motor, however, excitation current for generating a magnetic flux is generally weakened in order to suppress heat generation of the main shaft motor (induction motor), at the time of a light load. When power failure occurs on the AC power supply side in a state that the excitation current is weakened and the kinetic energy of the feed shaft is large, it is not possible to promptly control acceleration and deceleration of the main shaft motor at a maximum output. Therefore, when the main shaft motor is an induction motor, it may not be possible to suppress a sharp rise or fall in the DC voltage of the DC link at the time of power failure on the AC power supply side.